1. Technical Field
The present invention relates in general to electrostatic discharge protection for an integrated circuit card. In particular, the present invention relates to a method and system for dissipating electrostatic energy during insertion of a circuit card into an electronic system. Still more particularly, the present invention relates to a method and system that allow gradual release of electrostatic energy when a circuit card is inserted into an electronic system, such as a computer chassis.
2. Description of the Related Art
An electrostatic charge or static electricity accumulated on a human can reach levels of over 1000 volts. Modern computer circuitry utilizes voltages lower than 1.5 volts in a digital logic circuits. Consequently, the transfer of a static charge from a human or a circuit board into computer circuitry can cause serious damage to electronic circuitry. Such a static charge transfer is commonly referred to as "electrostatic discharge" or "ESD". Protection of electronic circuits from ESD has been an onerous problem since the advent of the transistor.
An electrostatic charge can build up on the surface of a person's body in many ways, including friction against surfaces such as carpet (i.e., triboelectric charging). When the fingertip of a charged human comes in close proximity to a conductor that is coupled to a circuit board, the accumulated charge rapidly discharges through every circuit coupled to the conductor. In such an ESD scenario, many sensitive circuits can be irreversibly damaged. Complementary metal-oxide semiconductor (CMOS) circuits and integrated circuits having small geometries are now widely utilized in the home and office. Environmental conditions in the home and office subject sensitive circuits to electrostatic discharge by humans. Heating and melt down due to electrostatic discharge presents serious reliability problems for deep sub-micron CMOS technology utilizing silicon junctions for ESD protection. As manufacturers attempt to increase the reliability of electronic equipment, prevention of ESD damage has become a primary concern.
Aside from physical damage to electronic components, an additional problem concerning ESD arises when a circuit card with an accumulated electrostatic charge is inserted into an activated electronic system. An increasing number of electronic systems are designed to be activated at all times. A categorical example of such a full-time operational system is a computer network server system. Such systems are designed such that circuit cards may be added, removed, or serviced while the system is running. In addition to the danger of physical component damage, the operations of an activated system may be significantly disrupted due to electrical transients resulting from an ESD that occurs when a circuit card is inserted into such a system.
A common method of providing ESD protection is simply to ensure that the circuit card, and the person inserting the card, are both grounded prior to and during card insertion. Grounding the circuit card may be accomplished by utilizing a "grounding strap" to electrically connect the system chassis to the person in contact with the card, such that electrostatic charge will not accumulate on the card. Such a method of ESD protection, if implemented alone, leaves a substantial risk of human error or neglect. A user either may not properly attach the grounding strap or may fail to utilize the strap at all, thus leaving the circuitry aboard the circuit card vulnerable to ESD damage and also risking a disruption in the electrical operation of the system into which the card is inserted. Other methods of ESD protection such as utilizing grounding mats are similarly deficient.
A wide variety of designs for accomplishing on-chip ESD protection have been developed. The primary function of an ESD protection circuit is to direct an ESD away from the circuit to be protected. One such method implemented to protect electronic circuits from ESD damage is to incorporate filters on the input of integrated circuits. ESD filters are typically comprised of resistors and diodes or any combination thereof. FIG. 1 depicts a typical ESD filter circuit which utilizes resistors and reverse-biased diodes for de-coupling ESD transients.
The resistance and the inherent capacitance associated with reverse-biased diodes and other components significantly slow down the attainable data transmission speed of an integrated circuit. Therefore, state-of-the-art microprocessors cannot attain data transmission and reception of digital data on the order of hundreds of megahertz to one gigahertz due to the degradation of parasitic characteristics inherent in existing ESD filter designs.
Traditional on chip ESD protection circuit designs, such as that depicted in FIG. 1, involve a trade off between increased capacitance on the input/output (I/O) transmission line and the transient voltage dissipation capacity.
From the foregoing it can be appreciated that there is a need for improved electrostatic discharge protection when a circuit card is plugged-in and unplugged from a computer chassis.